We lived in the Charente for 4 years, and now have returned just into the south Vienne, and to be honest, most of the large circle we have moved in probably wouldn't have a garden designer for their plots...Money is tight for ex-pats here already, landscaping materials can be very costly, as are plants, and it can be a real challenge to have a 'garden' here concerning watering, heat, etc. (we always ensure we buy a property with own water source, eg). Also there is so much web advise for designs that would 'fit' your plot already out there, unless you also mean you'd be 'hands on' getting stuck into the actual works, too? Just sourcing reasonably priced good top-soil can be a real pain here, as we've found. We've created 3 gardens since arrival, about to do a fourth, and part of the fun/challenge has been to source plants as and when cheaply that will do well, to create our own designs after living a full 4 seasons (getting to know soil, heat, wind, drainage, access, etc,), to know which builders merchant provides the best products at the most competative rates, and to split and propogate those said plants. We haven't yet found a need for an actual designer to advise, and without in any way at all wishing to be rude, if we were to consider at any point that route, we would be using an established designer here that has lived in the region a good while and knows its many vagaries.
Sorry if I seem to be pouring cold water on plans, but, believe me, we've seen many a dream go out the window along with all monies for ex-pats for many an idea/scheme. I'd urge you to really do in-depth research, look into competition (of which there is) and then live in your chosen region a year or two at least, before committing any savings to what may turn into a nightmare scenario.
And, of course, it's generally understood France is also a nightmare to run a successful business in in most cases....but that's another story/hurdle! I wish you all the best.